I'm coming into this thread a bit late... but I certainly have some opinions!
I think a big thing this thread is lacking... is pictures!
But before that...
Now, the way I understand things from the original post, the original poster said, "The goal of the Generations line seems to be to update the designs of characters that we know of from previous lines." To me, this has certainly been the case when it comes to "Classics" style figures, and how they are as updates of G1 characters. Now, we all know that there is a whole lot of confusion as to what falls into "Classics," or what even to call them as -- technically -- "Classics" would only refer to the short-lived line that came out in 2006.
To me, any toy that can be used as an update of a G1 character, even if it is repurposed from another line, as long as it is from the 2006 Classics toyline or a later toyline, is a "Classics" verson of a character.
So, with that said, how well do different toys work as an update?
We'll start with the leaders.
1. Optimus PrimeStraight off of the card, I'd give it a 90%. Many people are bugged by the arm kibble... and yes, the original Prime didn't have that. Yes, it'd be nice if the weren't there. But they're not the end of the world to me. What does knock the score down is the lack of a trailer. But I only knocked 10% off for that, as I understand that trailers aren't the most practical thing in the modern toy market.
But if we're allowed to include 3rd party add-ons in our scores, if you give Optimus his Fansproject trailer and some Reprolabels, I'd give him 100%!

2. MegatronI want to be forgiving, as to me a "Classics" style Megatron needs to be a gun and it is obvious that it can't be a realistic type of gun. But that being said, I'm still only giving him a 70%. He'd move up to between 80% - 85% if he had the coloration of the Takara version.

3. OnslaughtThere's been some talk in this thread about Onslaught. As far as the FOC figure, I'll admit I don't own it... but I'd give him a 0%. Why is he even being talked about in a thread about toys that are updates of earlier figures? He's not an update of an older Onslaught. He's a new Onslaught.
For updates... well, we have two mold options. There's the Ultra Class Onslaught.


I'd give him an 80%. He looks good. He's a good toy. He has quite a different vehicle mode, but it is still identifiable as Onslaught. He looses some points because, while identifiable, it is significantly different. He also looses points because, well... he's not a combiner torso.
The other mold option would be the torso from the Eneron Bruticus Maximus combiner. Not every version of that mold... The original version of it was a homage to Onslaught, but was a new and unique character. But some of the more recent uses of the mold have been intended to be updated versions of the G1 character. The one I have is the Asian Exclusive version.
Out of the box, I'd give him 80%. Yes, he homages the original version... but not terribly well. Both modes have at best superfluous resemblance to the original. If not for the fact that he was a combiner, I'd have scored him even lower.
But once again, some third party accessories come to the rescue. With those, I'd give this Onslaught a 90%. The vehicle mode becomes perfect! The robot still isn't the best... He could really use a new head sculpt. But he is bulked up and made sufficiently more bad-arse to the point where I do like it better.

(I don't want to get into Bruticus too much, since my version includes two figures that are completely third-party... But I'd give that Bruticus 100%!)
And to even things out, one more Autobot... Well, two, since they use the same mold.
4. & 5. Tracks and WheeljackThis mold seems to get a lot of hate. Specifically, the use of this one mold to represent these two characters who had different molds in the G1 days. But you know what? I'm going to give them each a 95%... and bump that up to 100% when you add Reprolabels to them.

There's enough remolding to give each one unique details to the vehicle modes, and those mold differences plus differences in the transformation process give each robot their own look, too.

Suppose I'll end there for now.